When petroleum well bores are drilled to the desired depth for production, the well operator determines whether the well bore should be placed into production or abandoned. A bore hole cuts through the various rock layers in the formation through which it is drilled and provides a route between the numerous porous zones of the formation. The bore hole allows fluids to migrate either upwardly or downwardly in between zones. Such interzonal migration must be controlled.
When it is determined that the well bore should be placed into production, a casing is run into the well. The casing is a length of tubing usually formed of steel. To prevent migration of fluids through the annulus between the casing and the well bore wall, a cement slurry is often pumped into the annulus during well completion. The cement is placed in sufficient volume to displace the drilling fluid in the annulus and is intended to prevent interzonal migration of fluid outside of the casing. However, fluid migration outside of the casing sometimes occurs through voids formed during set up of the cement or due to cement decomposition. Because the cement, when set, is solid, any voids formed in the cement during set up are permanent.
In some drilled bore holes, it is determined that the hole is not suitable for production. Such a bore hole is abandoned without running casing. Cement is also used in open boreholes to plug and abandon the well. Voids may also form during the set up of the cement in an abandonment plug.
Problematic interzonal migration occurs through voids in the cement seals used for completion and abandonment of boreholes.